Some years ago I picked up Nomad travel blanket, basically a synthetic-filled square quilt useable as a sleeping bag, blanket, insulated serape or poncho liner (and indeed it's very similar to the old US Army poncho liners you can find in surplus stores: those might be worth looking at as they're quite cheap). The current equivalent looks to be the "Travelproof Variant III" which you can see at this link: http://upthear.se/1zd Prices have definitely risen, though.
It's not the most thermally efficient system and, being square it weighs a bit more than an equivalently warm mummy bag, but as others have pointed out a typical sleeping bag would probably be far too warm anyway. With this thin blanket it's easy to open it up in order to cool off, but it does provide a bit of insulation when needed, assuming that even this amount of warmth won't be too much.
Edited: 03/02/2011 at 18:09